1. Field of the Invention
The present invention relates to a metal dome switch for a keypad, and more particularly, to a metal dome switch for a keypad in which an apex of a dome-shaped metal plate can be normally elastically deformed in a concave shape to always accurately perform a switching function and click sensitivity can be optimally maintained, even when a center part of a key button installed in a keypad of a mobile terminal, etc., is inaccurately pushed.
2. Discussion of Related Art
In general, as shown in FIGS. 1 and 2, a keypad used in a mobile terminal such as a mobile phone includes upper and lower cases 1 and 2 vertically spaced apart a predetermined distance from each other, a printed circuit board 3 mounted on the lower case 2 and having an upper surface on which connection terminals 3a are formed, a plurality of metal dome switches 4 disposed on the printed circuit board 3 to perform a switching function by selectively contacting and being spaced apart from the connection terminals 3a, a spacer 5 provided on the printed circuit board 3 and on which the metal dome switches 4 are installed, a tape 6 adhered to the spacer 5 to fix the metal dome switches 4, a light guide plate 7 installed on the metal dome switch 4 to emit light received from a lamp (not shown) to a surface thereof, an upper sheet 8 disposed on the light guide plate 7 and made of a synthetic resin, and key buttons 9 installed in the upper case 1 to press and elastically deform the metal dome switches 4.
Therefore, as a user pushes and releases the key button 9 with his/her finger, as shown in FIG. 3A, the apex of the metal dome switch 4 is elastically deformed in a concave shape to contact and be spaced apart from the connection terminal 3a, performing a switching function.
In addition, a pressure concentration projection 8a is formed on a lower surface of the upper sheet 8, on which the metal dome switch 4 is disposed. The pressure concentration projection 8a is configured such that a pressure applied by a push operation of the key button 9 is concentrated to the apex of the metal dome switch 4 to normally elastically deform the metal dome switch 4, accurately performing a switching function and improving click sensitivity upon the push operation of the key button 9.
However, in the conventional keypad for a mobile phone, as shown in FIG. 2, only when a user accurately pushes the center of the key button 9 in an arrow direction a, the pressure concentration projection 8a of the upper sheet 8 presses the center of the metal dome switch 4 in an arrow direction a′. As a result, as shown in FIG. 3A, the apex of the dome-shaped metal plate is elastically deformed downward to perform a normal switching function, improving the click sensitivity. However, when a user carelessly pushes a peripheral part of the key button 9 other than the center thereof in an arrow direction b or c, the pressure concentration projection 8a of the upper sheet 8 also presses a peripheral part of the metal dome switch 4 other than the center thereof in an inclined arrow direction b′ or c′.
In this case, as shown in FIG. 3B, since the apex of the metal dome switch 4 has a dome-shaped curve, a portion of the metal dome switch 4 to which the pressure is applied is elastically deformed to a large extent, and an opposite portion to which the pressure is not applied is elastically deformed to a small extent.
Therefore, the apex of the metal dome switch 4 is elastically deformed while leaning to one side so that the apex is not in contact with the connection terminal 3a to cause malfunction thereof, making a user cumbersomely push the key button 9 again. In addition, even when the metal dome switch 4 is in contact with the connection terminal 3a to perform the switching function, click sensitivity may be remarkably decreased.
Moreover, when the upper sheet 8 moves in the keypad and thus the pressure concentration projection 8a is deviated from the center of the metal dome switch 4 even to a small extent, it is difficult to accurately push the center of the key button 9 to normally elastically deform the apex of the metal dome switch 4 in a concave shape, decreasing the switching function. In order to solve the problem, in the conventional art, while the upper sheet 8 is securely fixed to the printed circuit board 3, etc., to prevent movement of the upper sheet 8, a process of assembling the keypad becomes cumbersome, and assemblability and productivity are also decreased due to addition of the assembly process.
Such problems could be solved by, as shown in FIG. 4, removing the pressure concentration projection formed at the upper sheet 18 and providing a pressure concentration projection 14a projecting upward from an apex of a metal dome switch 14 so that a pressure of a key button 19 is concentrated to the apex of the metal dome switch 14 through the pressure concentration projection 14a. 
The metal dome switch 14 must have the pressure concentration projection 14a having a predetermined height sufficient to maintain the click sensitivity in an optimal state upon a push operation of the key button 19. However, since the height of the pressure concentration projection 14a is limited, the click sensitivity cannot be maintained in an optimal state.
The conventional metal dome switch 14 is formed of a thin stainless steel plate having high elasticity. As shown in FIG. 5A, the pressure concentration projection 14a includes an uplift part 14a-1 projecting from the dome-shaped metal plate, and a pressing surface 14a-2 horizontally extending from the uplift part 14a-1. The pressure concentration projection 14a is formed by a press forming process such as a bending process or a half-blanking process in which a clearance c between a punch diameter and a die hole is set to a very small size.
The bending process and the half-blanking process, which are similar press forming methods, are distinguished in that clearances c between a punch diameter and a die hole are set to 0.3 to 1 t and 0.03 to 0.1 t with respect to a thickness t of each material. When a small projection such as the pressure concentration projection 14a is formed at the metal dome switch made of a stainless steel material having higher elasticity than that of another press forming method, it is possible to accurately form the pressure concentration projection 14a at a desired position while minimizing deformation in outer appearance and dimension of the metal dome switch. However, it is difficult to apply a press forming process such as a drawing process of forming a soft metal material to form the pressure concentration projection 14a of the metal dome switch made of a stainless steel material having high elasticity, because deformation in outer appearance and dimension of the metal dome switch becomes severe.
However, as shown in FIGS. 5A and 5B, in the case of the bending process or the half-blanking process, a thickness a of the uplift part 14a-1 is reduced in proportion to a height h of the pressure concentration projection 14a to be formed. That is, as shown in FIG. 5A, when the height h of the uplift part 14a-1 is formed to ½ t or less of a thickness t of a metal plate, variation in thickness b of the uplift part 14a-1 is reduced to maintain structural strength thereof to some extent. However, as shown in FIG. 5B, when the height h is formed to ½ t or more of the thickness t of the metal plate, the thickness a of the uplift part 14a-1 is also reduced. In this case, since the structural strength of the uplift part 14a-1 is weakened and severe cracks are generated, such a pressure concentration projection 14a-1 cannot be applied to a keypad due to characteristics of the metal dome switch that must be repeatedly operated hundreds of thousands of times. In particular, in the case of the half-blanking process in which the clearance between the punch diameter and the die hole is set to a smaller size than that of the bending process, the thickness a of the uplift part 14a-1 is further reduced to weaken the structural strength, and in severe cases, the uplift part may be broken, making it impossible to apply the pressure concentration projection to the keypad.
Therefore, since it is difficult to form the height of the uplift part 14a-1 of the pressure concentration projection 14a to ½ t or more of the material thickness through the conventional press forming process such as the bending process or the half-blanking process, the projection thickness of the pressure concentration projection 14a is too small to accomplish optimal click sensitivity upon the push operation of the key button.